1. Field of the Invention
The present invention relates to a liquid crystal display device and more particularly, the present invention relates to a liquid crystal display device of a pixel-divided type.
2. Description of the Related Art
A liquid crystal display (LCD) is a flat-panel display that has a number of advantageous features including high resolution, drastically reduced thickness and weight, and low power dissipation. The LCD market has been rapidly expanding recently as a result of tremendous improvements in its display performance, significant increases in its productivity, and a noticeable rise in its cost effectiveness over competing technologies.
A twisted-nematic (TN) mode liquid crystal display device, which used to be used extensively in the past, is subjected to an alignment treatment such that the major axes of its liquid crystal molecules, exhibiting positive dielectric anisotropy, are substantially parallel to the respective principal surfaces of upper and lower substrates and are twisted by about 90 degrees in the thickness direction of the liquid crystal layer between the upper and lower substrates. When a voltage is applied to the liquid crystal layer, the liquid crystal molecules change their orientation direction to a direction that is parallel to the electric field applied. As a result, the twisted orientation disappears. The TN mode liquid crystal display device utilizes variations in the optical rotatory characteristic of its liquid crystal layer due to the change of orientation directions of the liquid crystal molecules in response to the voltage applied, thereby controlling the quantity of light transmitted.
The TN mode liquid crystal display device allows a broad enough manufacturing margin and achieves a high productivity. However, the display performance (e.g., the viewing angle characteristic, in particular) thereof is not fully satisfactory. More specifically, when an image on the screen of the TN mode liquid crystal display device is viewed obliquely, the contrast ratio of the image decreases significantly. In that case, even an image, of which the grayscales ranging from black to white are clearly observable when the image is viewed straightforward, loses much of the difference in brightness between those grayscales when viewed obliquely. Furthermore, the grayscale characteristic of the image being displayed thereon may sometimes invert itself. That is to say, a portion of an image, which looks darker when viewed straight, may look brighter when viewed obliquely.
To improve the viewing angle characteristic of such a TN mode liquid crystal display device, one pixel electrode may be divided into multiple sub-pixel electrodes such that the voltage to be applied to the pixel electrode is distributed to those multiple sub-pixel electrodes at a predetermined ratio as proposed in Japanese Laid-Open Publication No. 6-332009, for example. Such a technique will be referred to herein as a “pixel dividing technique”. In the arrangement disclosed in Japanese Laid-Open Publication No. 6-332009, a control capacitor electrode is provided so as to face multiple sub-pixel electrodes with an insulating layer interposed between them and to apply voltages to the sub-pixel electrodes at a predetermined ratio by forming capacitors to be coupled with the sub-pixel electrodes.
In that arrangement, however, the voltages being applied to the sub-pixel electrodes are changeable due a variation in the thickness of the insulating layer that is provided between the sub-pixel electrodes and the control capacitor electrode. Thus, it is difficult to control the voltages being applied to the sub-pixel electrodes at the predetermined ratio.
Also, in the arrangement described above, the higher the voltages being applied, the greater the potential difference between the sub-pixel electrodes. The present inventors discovered and confirmed via experiments that the application of such voltages effectively improved the viewing angle characteristic of a liquid crystal display device operating in a display mode that shows heavy viewing angle dependence responsive to relatively high voltages applied (e.g., a normally white mode TN mode liquid crystal display device) but could not improve the viewing angle dependence of the γ characteristic of a normally black mode liquid crystal display device sufficiently.
Examples of normally black mode liquid crystal display devices include an in-plane switching (IPS) mode liquid crystal display device (see Japanese Patent Gazette for Opposition No. 63-21907), a multi-domain vertical aligned (MVA) mode liquid crystal display device (see Japanese Laid-Open Publication No. 11-242225), and an axisymmetric aligned (ASM) mode liquid crystal display device (see Japanese Laid-Open Publication No. 10-186330), all of which were developed relatively recently as TN mode liquid crystal display devices with improved viewing angle characteristics. In a liquid crystal display device operating in each of these newly developed wide viewing angle modes, even when an image on the screen is viewed obliquely, the contrast ratio never decreases significantly or the grayscales never invert unlike the old-fashioned TN mode liquid crystal display devices.
However, as the display quality of liquid crystal display devices has been further improved recently, the viewing angle dependence of the γ characteristic surfaced as a non-negligible problem to resolve in order to improve the viewing angle characteristic. That is to say, a liquid crystal display device may exhibit different γ characteristics when an image on the screen is viewed straightforward and when the same image is viewed obliquely, respectively. As used herein, the “γ characteristic” means the grayscale dependence of display brightness. That is to say, if the γ characteristic in the frontal direction is different from that in the oblique direction, then the grayscales sensed in the frontal direction will be different from those sensed in the oblique direction. Thus, this problem is particularly non-negligible when an image such as a still picture is displayed or when a televised video is displayed.
The viewing angle dependence of the γ characteristic is more remarkable in the MVA mode or in the ASM mode than in the IPS mode. On the other hand, when the IPS mode is adopted, it is more difficult to manufacture panels showing a sufficiently high contrast ratio to the front viewer than the MVA mode or ASM mode. In view of these considerations, the viewing angle dependence of the γ characteristic of the MVA or ASM mode liquid crystal display device should be improved.